Interim Labor leader Chris Bowen says he won’t be supporting any move to disallow MPs a conscience vote on the issue of same-sex marriage.

Australian Workers Union national secretary Paul Howes on Wednesday called for Labor to have a “nasty and divisive” internal debate about doing the “right” thing on same-sex marriage, ahead of a speech at a Sydney forum in which he will call for the party to change its rules to force MPs to vote in favour of marriage equality.

Mr Bowen told Sky News: “I actually think there’s a case for more ability for caucus members to vote on issues according to their views, not less.”

“So I would not support a move away from a conscience vote, and I think this matter was properly aired at the last national conference.”

Labor’s current position on the issue, which allows a conscience vote, was adopted after a fractious debate at its last national conference in 2011.

Mr de Bruyn, whose controls one of the largest right-wing unions in the country, told The Australian Financial Review that the ALP had adopted a conscience vote and he “couldn’t see that changing”.

“This was decided two years ago, it’s settled. Both the leadership contenders support that and I don’t see a change happening as the Labor Party has a long history of allowing conscience voting,’’ he said.

“His comments are strange and unusual.”

If support for same-sex marriage were to be made a binding matter, there would be severe consequences for Labor MPs who crossed the floor when it was put to a vote in parliament.

Mr Howes acknowledged some in the union movement and the party were vehemently opposed to gay marriage but “frankly that’s irrelevant when you actually look at this issue from a matter of principle about what is right and wrong”.

“It’s wrong for us to put this down as some issue which is hard for the party to have a definitive position on,” Mr Howes told ABC radio ahead of his speech.

Mr Howes, who was rumoured to be eager to replace Bob Carr in the Senate but has ruled himself out, said the case for marriage quality was as clear cut as that for racial equality.

Therefore, Labor had to have the discussion, “even if it does mean some pretty nasty and divisive fights internally”, he said.

“We arrived at this decision at the last national conference because we didn’t actually want to have this discussion internally about how Labor does politics.

“But one of the things I’ve been thinking about is we’ve had a lot of discussion over the last three weeks about the way that we elect the leader and the membership of the party having a say.

“I reckon if we polled this question with the membership of the Labor party we would be talking about a comprehensive and significant majority of party members would support the position that I’m advocating today.”